The present invention relates generally to acceleration sensors and more specifically to acceleration sensors of the type adapted for use in an automotive vehicle equipped with an inflatable passenger restraint or airbag. To operate an inflatable occupant restraint system in an automotive vehicle, it has been found most desirable to provide one or more sensors positioned in the vehicle that respond to changes in the vehicle's velocity to transmit an electrical signal to operate the inflating device. One type of such sensor found to be functionally acceptable is a sensor having an acceleration sensing mass on which a biasing force is imposed by a permanent magnet. The mass is moved in response to the occurrence of an acceleration pulse at a level above a predetermined level to a position in which it closes a switch to operate the inflatable restraint device. Magnetic force is used to hold the mass in its inactive position and movement of the mass is fluid damped to identify accelerations of sufficient magnitude and duration to make inflation desirable by controlling the peripheral clearance between the mass and the structure surrounding it as it moves in its path to close the switch. U.S. Pat. No. 4,329,549 to Breed is exemplary of such sensors. One alternative to such designs is the substitution of a spring mechanism for the magnet in biasing the acceleration sensing mass to its inactive position. Exemplary of such designs is that shown in U.S. Pat. No. 4,284,863 to Breed.
While functionally acceptable, the known sensors suffer certain disadvantages which adversely affect the cost of their manufacture. Chief among these are the necessity to closely control peripheral tolerance between the mass, which is generally formed as a Precision ball, with respect to a metallic housing or sleeve in which is formed a bore along which the ball travels. Expensive plating, honing and selective assembly operations are sometimes necessary to assemble acceptable sensors.
Another disadvantage, in part related to the requirement for closely controlling tolerances between acceleration mass and housing or sleeve, is the expense attendant the need to compensate for differential thermal expansion between parts. This has required the use of expensive and difficult to machine materials, and the provision of certain materials and some mechanisms for sealing the sensors such as potting which do not lend themselves well to automatic assembly techniques.